This invention relates to switches and more particularly to a high power RF (radio frequency) switch.
Prior art switches intended for use with high frequency at power levels of 10 KW (kilowatts) and greater have been of three general types. The three general types of switches are coaxial, vacuum and motor driven plunger or knife switches.
The coaxial category of switches are intended for use with coaxial transmission lines. Due to the nature of their intended application, such switches involve distributed capacitance and inductance values consistent with the impedance and power ratings of the transmission lines with which they must conform. These values of capacitance and inductance are not suited to applications involving RF impedance levels significantly higher or lower than that of the characteristic impedance of the lines for which the switch was designed. This impedance is generally 50 ohms. Additionally, such switches are generally limited in RF voltage and current capabilities consistent with the transmission line they were intended to interface with. The application of such switches is, in general, limited to use with the specific transmission lines for which they were designed.
The category of vacuum switches includes those which have contacts mounted within an evacuated envelope. They may be actuated by either an electromagnetic solenoid, pneumatically, or with a cam mechanism. The contacts take the form of two conducting rods, one fixed and one movable. The movable rod is displaced along its axis to bring it into contact with the fixed rod. The vacuum seal is maintained by mounting the movable rod to a bellows which moves with the rod. The use of such bellows limits the maximum practicable travel of the movable contact rod. In order to limit the amount of flexure, the bellows must be limited to travel only a small percentage of their overall length. As a result, the total electrical path through the switch must be long with respect to the gap achieved when the switch is open. This is incompatible with low intercontact capacitance and low switch inductance.
The third category of switches for high power RF application are motor operated plunger or knife switches. These switches are either conductive rods which are driven through contact rings or blades with the rods being driven between the contacts. In either case, the large travel required for high voltage applications results in operating times of several seconds or in complex mechanical systems with their inherent poor reliability. As with the coaxial and vacuum types, the plunger or knife switches generally result in relatively large values of stray capacitance and inductance.
In applications where the values of stray capacitance and inductance are critical limits to the practicability of a design, the above mentioned switches are unacceptable. In the development of high power RF switches, five parameters are of importance. These parameters are: (1) capacitance between open switch contacts, (2) capacitance from the contacts to ground, (3) inductance of the active path through the switch, (4) speed of actuation and (5) reliability. None of the types of switches mentioned hereinabove meet these five requirements.